The present invention relates to packaged microelectronic elements and methods of fabricating them, and more particularly, to a stackable packaged microelectronic die assembly.
Microelectronic chips are typically flat bodies with oppositely facing, generally planar front and rear surfaces and with edges extending between these surfaces. Chips generally have contacts, sometimes also referred to as pads or bond pads, on the front surface which are electrically connected to the circuits within the chip. Chips are typically packaged by enclosing them with a suitable material to form microelectronic packages having terminals that are electrically connected to the chip contacts. The package may then be connected to test equipment to determine whether the packaged device conforms to a desired performance standard. Once tested, the package may be connected to a larger circuit, e.g., a circuit in an electronic product such as a computer or a cell phone, by connecting the package terminals to matching lands on a printed circuit board (PCB) by a suitable connection method such as soldering.
Microelectronic packages may be fabricated at the wafer level; that is, the enclosure, terminations and other features that constitute the package, are fabricated while the chips, or die, are still in a wafer form. After the die have been formed, the wafer is subject to a number of additional process steps to form the package structure on the wafer, and the wafer is then diced to free the individually packaged die. Wafer level processing may be a preferred fabrication method because it may provide a cost savings advantage, and because the footprint of each die package may be made identical, or nearly identical, to the size of the die itself, resulting in very efficient utilization of area on the printed circuit board to which the packaged die is attached. A die packaged in this manner is commonly referred to as wafer-level chip scale package or wafer-level chip sized package (WLCSP).
In order to save additional space on the substrate to which a packaged die is mounted, multiple chips may be combined in a single package by vertically stacking them. Each die in the stack must typically provide an electrical connection mechanism to either one or more other die in the stack, or to the substrate on which the stack is mounted, or to both. This allows the vertically stacked multiple die package to occupy a surface area on a substrate that is less than the total surface area of all the chips in the package added together. Because there are in general far more electrical connections when using a die stack than when packaging a single die, the electrical connections between the various dies of the stack must be extremely robust and reliable.